1. Field of the Invention
This invention relates to a thermal stencil master sheet for stencil printing and more particularly it relates to a thermal stencil master sheet obtained by adhering a thermoplastic synthetic resin film onto a porous substrate and an adhesive therefor.
2. Description of the Related Art
Heretofore there has been known a thermal stencil master sheet obtained by laminating a thermoplastic resin film onto a porous substrate such as a porous thin paper with an adhesive and providing a releasing agent layer on the surface of the film for preventing meltadhesion thereof onto manuscripts. Examples of the thermoplastic synthetic resin film used for such a master sheet are vinylidene chloride-vinyl chloride copolymer films, films of copolymers composed mainly of propylene, polyester films, etc. Further, examples of the porous substrate used are thin paper obtained by interlacing natural fibers or chemical fibers, sheets, non-woven fabrics, etc. Further, examples of the adhesive with which they are laminated are vinyl acetate adhesives, acrylic adhesives, rubber adhesives, etc.
As for the properties of the adhesive according to the known art, the following specific feature is required:
(i) carrying out the coating process rapidly. Further, for the properties of the adhesive layer after preparation of the base paper,
(ii) the following specific features are required:
When the master sheet is laid on a manuscript and infrared lamp light or flash light is irradiated thereon to form a perforated printed image, the adhesive layer melts together with the thermoplastic resin film (i.e. the layer is superior in the so-called heat-sensitivity) and (iii) after perforation of the master sheet, when printing is repeated, the adhesive layer is not damaged by the solvent contained in the printing ink and hence is superior in the adhesion strength (i.e. superior in the durability against printing)
However, since the adhesives so far used for producing master sheets have had a large quantity of solvents and dispersing agents blended therein, a long coating rate such as at a rate of about 10-20 m/min is required, so that the production efficiency is notably reduced. Further, since the solvents used are volatile, they are liable to cause fire, air pollution, etc. Therefore, equipment has been required for recovering the solvents.
Further, vinyl acetate adhesives which have been most broadly used due to their easy handling are insufficient in their adhesion strength. For example when the same printed image is printed over two days, the stencil master sheets perforated by thermal perforation are allowed to stand on a printing machine overnight in a state where they are in contact with the printing ink during which time the vinyl acetate adhesive layer is gradually damaged, and when the image is printed after the lapse of one night, it becomes unclear; hence there is a drawback that it is impossible to achieve an objective number of sheets to be printed.
Thus the present inventors have made extensive research in order to overcome the above-mentioned drawbacks, and as a result have found that when a specified urethane prepolymer is used for the urethane adhesives which have so far been regarded as difficult to use, the coating rate becomes as high as 5 to 10 times that of the conventional process. The adhesive layer after curing exhibits a good perforation (heat-sensitivity) through conventional thermal perforation means such as infrared lamp, flash lamp, etc.; and the adhesive layer is not damaged by the solvent contained in a printing ink. The adhesive is also superior in adhesion strength so that even when printing is carried out over two days or longer a printable stencil master sheet can be obtained.
Based on the above findings, the present invention has been completed.